1. Field of the Invention
The present invention relates to a shift control system for an automatic transmission.
2. Description of Related Art
In an automatic transmission for an automotive vehicle, generally, as shown in FIG. 6 the transmission is provided with reverse clutch 1 and low reverse brake 2 as frictional elements, which are brought into engagement with each other so as to obtain a rear shift stage for rearward movement. Such an automatic transmission is disclosed in Japanese Patent Public Disclosure (JP A) No. 63-186055, laid open to the public in 1988. Generally, the low reverse brake 2 is provided with a dish plate, as disclosed in Japanese Utility Model Publication (JU Y) No. 57-52426, published in 1982. The dish plate suppresses an undue pressure increase in the low reverse brake 2. The automatic transmission is provided with a main hydraulic line 5 into which a hydraulic pressure is introduced when a manual valve 4 takes a reverse position. The main hydraulic line 5 is divided into a first line 6 and a second line 7. In this case, the first line is connected with the reverse clutch 1 while the second line 7 is connected with the low reverse brake 2.
Accumulator 8 is disposed on either the first line 6 or the second line 7. In this structure, the hydraulic pressure is directly supplied to one of the frictional elements, for instance, low reverse brake 2, to accomplish a quick engagement, but indirectly supplied to the other of the frictional elements, for instance, reverse clutch 1 through the accumulator 8 to make a gradual engagement and to thereby prevent a shift shock.
On the other hand, when the manual valve 4 is switched to another range, such as the N-range, the hydraulic fluids in the frictional elements 1 and 2 are drained through drain port 4a of the manual valve 4 to release the frictional elements 1 and 2.
However, when the hydraulic fluids in both the frictional elements 1 and 2 are drained through the drain port 4a, the hydraulic fluid reserved in the accumulator 8 is also drained because the accumulator 8 is disposed on the hydraulic line which is communicated with the one frictional element. As a result, releasing timing of the one frictional element is delayed.
In other words, the other frictional element 2 is released prior to the one frictional element. When the reverse stage is established, the power transmitting unit, which includes the engine and transmission integrally connected to each other, transmits driving torque to the wheels for reverse movement. The power transmitting unit is subjected to a reactive force from the wheels to deform engine mount members. When the other frictional element is released as a so that the two frictional element is released at different timings when the reverse range is switched for another range, the power transmitting unit is made free from the dampings action of the engine mount members so that the power transmitting unit is shaken due to the resilient return action of the engine mount members. This is called a shift shock.